At that temp I'd allow a lot more time - at least 1.5 hours/# or even 2 hrs - actually at 225, more towards 2 hours/#. I haven't cooked at that temp in forever so I'm not really sure to be honest.
You could kick the heat up and get it done more quickly without losing any quality. I do 'em anywhere from 270-325 with consistently excellent results. I like to rest the cut at least an hour before pulling as well.
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